We have studied microscopic effects leading to mode competition in an inhom
ogeneously broadened medium of a gas laser with a weakly anisotropic cavity
. The polarization bistability observed in some lasers, which is believed t
o be caused by hole burning, we attribute to microscopic coherent effects i
nduced by two light fields. Due to these effects one field can be coupled w
ith another. Hole burning is found to be important, but not crucial. The ex
act solution of the equation of motion for field amplitudes is compared wit
h the third order approximation of perturbation theory. The role of four wa
ve mixing, important for bistability of axial modes in a homogeneously broa
dened medium, is discussed.